<title>Periodically switched erbium-doped fiber laser for enhanced temperature sensitivity in a fiber sensor</title>

Abstract

When a pulse of light is launched down a single-mode optical fiber, with a peak power below the threshold for nonlinear effects, backscattered Rayleigh and spontaneous Brillouin signals are generated. The Brillouin signal is temperature sensitive while the Rayleigh signal shows comparatively negligible sensitivity but serves as a backscattered signature for fiber attenuation and loss mechanisms. Ratioing one signal to the other extracts the temperature information. Optical separation of the Rayleigh and Brillouin signals of our sensor requires a source with bandwidth less than 90 pm. However, the narrowband Rayleigh signal generated is noisy due to coherent Rayleigh noise. A broader source generates a Rayleigh signal with reduced coherent Rayleigh noise. A Q-switched erbium-doped fiber-optic laser containing a fiber-optic switch in the laser cavity, which allows switching and selection of a narrow (<90 pm) or broad (~3 nm) bandwidth output coupler has been developed. Periodic switching should allow virtually simultaneous capturing of Brillouin and broadband Rayleigh signals thus ensuring that the spontaneous Brillouin signal is correctly referenced even if fiber attenuation, splice or bend losses should vary during extended data collection cycles.